In a typical beam index color television receiver, the cathode ray tube (CRT) has a single electron beam which scans color phosphor stripes provided on the display screen, and the scanning electron beam is modulated with the appropriate color information as the beam scans corresponding color phosphor stripes, in a process known as three color point sequential scanning. This differs from the conventional color CRT in which three individual electron beams simultaneously scan the color phosphor stripes with each beam being modulated with its associated color information.
A typical display screen of a beam index CRT has color phosphor stripes formed as triads of red (R), green (G) and blue (B) stripes, arranged successively in the horizontal scanning direction at a predetermined pitch, or spacing, so that the beam repeatedly scans RGB RGB . . . phosphor stripes. When the beam scans the R stripe, red color information is used to modulate the intensity of the beam, and correspondingly green and blue color information modulates the beam when it scans the B and G stripes, respectively.
In addition to the color phosphor stripes, a series of fluorescent or phosphorescent index stripes are provided on the display screen in a predetermined relationship with the triads of color phosphor stripes. As each index stripe is scanned by the beam, it emits light which is detected by a photodetector. The photodetector is responsive to the light emitted from the repetitively excited index stripes to generate an index signal in synchronism with the scanning of the electron beam having an index frequency as a function of the pitch of the index stripes and the horizontal scanning speed of the electron beam. Since the index stripes are in predetermined positional relationship with the triads of color phosphor stripes, the index signal may be used to control the switching of the color information at the appropriate times so as to modulate the beam with the red, green and blue information as the beam scans the R, G and B color phosphor stripes, respectively.
Since the index signal is generated in synchronism with the scanning of the electron beam, any non-uniformities in the scanning speed of of the beam introduce corresponding non-uniformities in the index frequency. These non-uniformities may arise from fluctuations in the transfer characteristics of the deflection yoke or other components used in the horizontal deflection circuit, from power source voltage fluctuations, from the influence of earth magnetism, or from other sources. If the horizontal deflection scanning speed of the beam is increased or decreased, the resulting change in the index frequency produces a phase shift in the times at which the red, green and blue information signals are switched to modulate the beam intensity. As a consequence of this disturbance in the timing of the color switching, color misregistration occurs. Thus, there is a definite need for apparatus to detect non-uniformities, in particular non-linearities, in the scanning speed of the electron beam and to compensate for such non-linearities in order to obtain proper color registration of the displayed video picture.
One method of reducing the deflection speed error is disclosed in U.S. Pat. No. 4,305,022, assigned in common with the present application, wherein compensation data for an entire frame of the video signal is stored in a memory and the deflection speed is compensated in accordance with the compensation data read out from the memory. This method, however, requires a very large memory for storing sufficient compensation data for an entire frame, and further requires complicated circuitry for writing in and reading out the compensation data to and from the memory. Furthermore, when the compensation data is preliminarily stored in a memory, there can be no compensation for real time changes taking place during the actual operation of compensation.
Another method has been proposed in U.S. Pat. No. 4,287,531, also assigned in common with the present application. In the apparatus disclosed in this patent, an oscillator generates an oscillating signal whose frequency is synchronized with the index signal, and a phase-locked loop, including a phase comparator for comparing the index signal to the oscillating signal so as to produce a control signal as a function of the difference therebetween, adjusts the frequency of the oscillating signal in accordance with this control signal. A switching arrangement is responsive to the oscillating signal to switch color information signals for modulating the beam. The apparatus also includes deflection control apparatus with a deflection device, such as an auxiliary yoke, for controlling the deflection of the beam, and a deflection signal generator which is responsive to the control signal produced by the phase-locked loop for supplying deflection control signals to the deflection device to vary the deflecting speed of the beam and thereby to maintain a substantially constant beam scanning speed. In this patented apparatus, as in the present invention, it is recognized that non-linearities in the horizontal deflection of the beam result in corresponding variations in the index frequency.
However, even with the improved deflection speed compensation provided by the apparatus of U.S. Pat. No. 4,287,531, perfect compensation for horizontal deflection linearity is impossible. This is because the compensation loop has an inherent delay time, which may be caused by inductance in the secondary deflection coil, in the low pass filter included in the phase-locked loop, or in other components. While this delay time is not large, it is yet not so small as to be negligible, nor can it be completely eliminated. This delay time causes a phase difference between the detected signal from the photodetector and the output signal from the secondary deflection coil, so that misregistration occurs.
Furthermore, when the scanning speed is linear, the control signal is a constant voltage, so that the deflection device may exhibit an oscillatory response, which is highly undesirable.